Best 'Comeback' Singles?

Man-eater was the one that sprung to mind for me too. When that dropped my God it was unreal. An absolute game changer.
Alison Moyet's 2002 comeback has already been mentioned, but in terms of chart action, new 90s dance sound and the video - as well as her new hair colour - "Whispering Your Name" was a rather interesting comeback in 1994 after a while away from the charts.

I guess with these things it can be more about perception as well.

Absolutely. I hate "Believe" being described as Cher's comeback as I cannot stand for "One By One" erasure, but I also know that outside of UK/Europe it's reasonably accurate. Likewise, I struggle to call "Frozen" a comeback, it's not like Madonna was not having hits for the previous eight years, but I get it, it was a return to her 1984-1990 heights.

Also cannot abide this new trend of "comeback" and "lead single" being interchangable terms.
Take That - Patience & The Flood

Yes, that was pretty remarkable. Patience surprisingly isn’t that much of a stand-out in their discography, but it was everything their comeback single had to be and it did its job better than anyone could have ever imagined. Of course, they bettered it with The Flood!
I don’t think they have it in them anymore to come back with such an epic single again, but I love a nice surprise.

Summer Moved On was a great comeback single for A-Ha, but there was even better stuff on the album (as well as on the other post-comeback albums).
Just going to drop this absolute banger. So huge it needed 2 versions.

I class this as a comeback as it was the first single from a new album in 10 years. In between it was remakes, remixes and greatest hits one off singles.